Conventional robots, either based on parallel kinematics or on serial kinematics, are formed by a series of mechanical actuators and structures with fixed dimensions, which limit the range of their work space to the maximum that can be reached by their articulations and structures. Larger work spaces require larger, heavier and more dynamic structures which involve oversizing the actuators, which finally complicates the automation of large-sized parts.
To alleviate these drawbacks, parts of the structure of robots have been replaced with cables in combination with a support surface. Cables are light-weighted and can be extended in large workspace. Tensioned cables can be used to apply pulling forces. That is why in most cable-based parallel planar robots, cables are responsible for the function of actuation. To achieve the planar constraints, different solutions exist.
German patent DE102010015530-B4 describes a cable-based planar motion mechanism having a vertical arrangement, in which the virtual plane can only be parallel to the gravity force. The mobile platform, which is actuated by several cables in the vertical plane, moves in the virtual vertical plane against gravity force. As a consequence, the mechanism has very poor resistance to perturbations coming from outside of the virtual plane.
U.S. Pat. No. 7,246,445-B2 refers to a cable-based planar mechanism in which a flat table is used. The mobile platform is located on this table and several cables are used to actuate the movement of the mobile platform in the plane that is parallel to the table. This arrangement offers a better unilateral perturbation than that of DE102010015530-B4, because the mobile platform can push against the table. Although U.S. Pat. No. 7,246,445-B2 indicates that the table may be aligned horizontally and may also run diagonally to the horizontal, up to a vertical position, in fact no means for keeping the mobile platform against the table during the moment has been mentioned.
It can therefore be understood from its description that it is only the portions of gravity force which keep the mobile platform against the table during the movement of the mobile platform. Thus, when pulling forces, for example, with magnitude of gravity, are exerted on the mobile platform, it is difficult to insure the planar movement of the mobile platform. It is worthy to notice that in U.S. Pat. No. 7,246,445-B2, one of the proposed embodiment of the table is described as with multiple capillary holes below the seat which are connectable to a partial vacuum source, in order to be able to suction the seat in place against the table after reaching the desired final position′. The mentioned capillary holes, when connected to a vacuum source, has the only function of fixing the position of the seat after reaching the desired final position, thus having no function of exerted forces on the seat during the movement for preventing it from being separated from the table to ensure a high dynamic planar motion of the seat alone the table.
International patent application WO2011148004-A1 partially solves the above-mentioned problems by providing a cable-based planar motion mechanism in which obliquely extended cables guarantee the movement of the mobile platform against a supporting table. As a consequence, the mechanism can work in any orientation, as the table together with the obliquely extended cables offer bilateral constraints to keep the movement of the mobile platform in a plane. The platform can work on inclined surfaces and even upside down surfaces thanks to an attraction system between the platform and the working surface. This attraction system is generated by a perpendicular component of the cables forces: since the cables have an oblique orientation with respect to the work surface, a force component is generated on the platform which constantly maintains it against the work surface. However, this implementation requires high tensions in the cables. Consequently, the power consumption of actuators is increased, requiring that much more powerful actuators be used in comparison the ones needed to solely generate the target movement.